Having had a remarkably wide-ranging career during which he headed companies involved in everything from small aircraft to shipbuilding to cement, Greenwich, Conn., businessman and philanthropist David W. Wallace has some difficulty arriving at a term that neatly sums up his profession. But when pressed, he finally settles on “lucky.” In a stroke of good fortune for the School of Medicine, Wallace and his wife, Jean McLean Wallace, have donated $6 million to endow two new professorships, adding